High surface area metal oxide particles are well known in the art. Conventionally they are precipitated from either a liquid or a gas phase. In the usual liquid phase embodiments, the acidity, temperature or concentration of the metal salt solution is varied to precipitate the metal oxide or hydroxide. Typical of that prior art process is the neutralization of an acid solution of aluminum nitrate or titanium chloride with sodium hydroxide. In the usual gaseous phase embodiment, a metal salt is evaporated and then hydrolyzed by steam or oxygen to precipitate the metal oxide. Representative of that prior art method are the formation of fumed silica from vaporized silicon tetrachloride and fumed alumina from vaporized aluminum chloride. In both the usual liquid and gas phase precipitations the particles formed can be quite fine and have quite large specific surface areas; however, these powders do not possess an internal foam structure.
The decomposition or hydrolyzation of metal salts in high temperature gas streams is common in the art. In the usual approach, known as the spray dryer technique, a slurry or solution of a decomposable salt is atomized into a gas. The particles so formed can be relatively large, hollow spheres. The spheres often contain holes where the residual water or decomposition gases have escaped from the interior of the particle. The hollow spheres formed under these conditions do not have high specific surface areas.
A method for high temperature hydrolysis of a metal salt solution is disclosed by Walsh in U.S. Pat. No. 3,273,962 and in an article entitled, "Ultrafine Metal Oxides By Decomposition of Salts in a Flame," Ultrafine Particles (Wiley & Sons, 1963). Two related U.S. Pat. Nos. 3,172,753 and 3,161,468, further describe Walsh's method. In this method, which Walsh terms "atomizing into a flame," a two fluid atomizing nozzle is used to form a cloud of solution droplets in a plume of gas. This plume may contain, or is subsequently combined with, the fuel and oxidizing gas for a flame. Subsequent to creation of the droplets, a pilot flame initiates combustion. When combustion occurs, the fuel gases are burned to provide heat and thus permit decomposition of the metal salts.
Walsh teaches the use of cool gases in a two fluid nozzle to atomize the solution with the heating occurring in the later combustion process. Two aspects of Walsh's method retard the rate at which the droplets are heated. Since the droplets are in a plume of the cool atomizing gas, they are to some extent insulated. Alternatively, if the atomizing gas is either fuel or oxidant for the flame, the droplet's evaporation will cool the gas, prolonging the time needed for the gas to reach ignition temperature, burn and release heat.
Walsh emphasizes that the processing temperature must remain below 1150.degree. C. in order to achieve hollow shelled particles. According to Walsh, when the reaction temperatures exceed 1150.degree. C. only thin platelets or wall segments remain. This occurs because the initial droplet heating is not sufficiently rapid in the Walsh method.
The Walsh method does not result in the formation of high surface area particles. The largest specific surface area taught by Walsh for alumina hollow shells is 24 square meters per gram and for titania is 10 square meters per gram. Walsh discloses the formation of zirconia but indicates that the particles will occur as flakes rather than as hollow spheres.
A Pennsylvania State University thesis by Renato Ciminelli entitled "Synthesis of Alumina from Al (NO.sub.3).sub.3 .9H.sub.2 O by the Evaporative Decomposition of Solution Process" examines the question of the decomposition of aluminum nitrate in a high temperature atmosphere. Ciminelli used an atomizing nozzle to spray aluminum nitrate solution into a furnace heated ceramic tube. Ciminelli found that aluminum nitrate droplets converted to aluminum oxide particles, with a hollow sphere or foam structure, when the temperature of the outside of the ceramic tube, midway along its length, was in a range from 700.degree. C. to 1000.degree. C. The formation of hollow spheres according to Ciminelli's method depends upon heat being added to the mixture of gas and droplets.
In Ciminelli's method, the solution droplets are heated relatively slowly. This occurs because the aluminum salt droplets first experience low temperature gases from the bifluid nozzle and the mixing between the spray plume and the hot furnace gas is slow. Further, the heat required to raise the temperature by the final 200.degree. C. to 400.degree. C. was conducted through the walls of the ceramic tube. Under these conditions, Ciminelli was unable to make alumina particles with surface area greater than 43 square meters per gram. Ciminelli also calcined the particles made in his apparatus. By further heat treating the alumina particles at 950.degree. C. he was subsequently able to raise their surface area to 77 square meters per gram. Specific surface area decreased drastically when higher calcining temperatures were encountered.